North Sea FPSO experience quells concerns about use in US Gulf

The Gulf of Mexico has unique characteristics that would have to be given careful consideration when considering the use of floating pro duc tion, storage and offloading (FPSO) units, though generally speaking these are comparable to challenges that have been successfully tackled when developing FPSOs for other parts of the world.
April 1, 1999
3 min read

CorrOcean's internal corrosion control technology is proving of increasing interest in the protection of equipment made of exotic materials, as shown by a recent delivery of a resistor controlled cathodic protection (RCP) system to the Seraya chemical plant complex in Singapore.

Altogether it designed and supplied 160 blind flange-mounted RCP anodes and 19 submerged RCP anodes to provide protection for 63 heat exchangers at the plant. Though constructed of 6Mo super austenitic stainless steel and 25%Cr super duplex steel, the heat exchangers had suffered from severe corrosion caused by high-temperature sea water, which is used as the main cooling medium.

Corrosion in exotic materials is becoming a more frequent occurrence as they are exposed to temperatures above their design range, says CorrOcean's director of marketing and sales, Kjell Wold. A major attraction of the RCP system is that it is applied to the existing piping system which does not have to be replaced. The system was supplied to Woodside's Goodwyn A and Cossack Piper installations in Australia at a cost which was only a fraction of replacing the existing piping.

The method, which uses a resistor in series with the anode to control both the potential of the stainless steel and the anode output, is based on the observation that the protection potential for preventing localised corrosion of stainless steel is more positive than the typical potentials offered by sacrificial anodes. The voltage drop over the resistor is therefore designed to obtain a sufficient, but not excessive, negative polarisation of the stainless steel.

The resistor control keeps the stainless steel in a protective potential range where the current requirements are very small. As a result, it is possible for a single anode to protect long lengths of piping at a very low anode consumption rate. Advanced computer simulation of cathodic protection coverage is an essential part of each installation.

Wold also draws attention to the benefits of incorporating RCP at the design stage for new plant. This makes it possible to use less costly materials like AISI 316 stainless steel in place of expensive exotic materials. According to one comparative study performed by CorrOcean for Norsk Hydro, the life-time cost of plant consisting of GRP piping and AISI 316 valves with RCP protection was calculated as being only 37% of the cost of using titanium throughout.

There are other advantages, such as the greater availability and shorter delivery times for the less exotic materials, and the greater ease of finding workers qualified to work with them.

Copyright 1999 Oil & Gas Journal. All Rights Reserved.

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